In recent years, inkjet printers have been used not only for household printing applications but also for business printing applications for offices and retail photos or industrial applications such as electronic circuit drawing and flat panel display production, and thus, the applications of the inkjet printers are spreading. Of those, a head of an inkjet printer for business is required to have high-speed printing performance, and in order to meet the requirement, ink ejection is performed at a higher frequency. Alternatively, in order to realize high-speed printing, a full-line head is used in which the width of a recording head is matched with that of a recording medium, and ejection orifices in a larger number than that of the conventional ones are arranged. In general, the full-line head is configured in such a manner that multiple recording element substrates are arranged on a support member.
In general, as an ink ejection method for a liquid ejection head, there are a thermal system and a piezoelectric system. The thermal system involves boiling ink by applying heat thereto to utilize bubbling force caused thereby, and the piezoelectric system uses deforming force of a piezoelectric element. In the case of the thermal system, temperature changes due to the heat generated during ejection, which influences image quality. The reason for this is as follows. When the temperature of a head rises, the temperature of ink also rises. The ejection amount of ink changes in accordance with the rise in temperature of the ink, and as a result, the printing density in an initial stage of printing becomes different from that in a later stage. On the other hand, in the case of the piezoelectric system, a change in temperature of ink caused by an ejection operation is small. Therefore, the image quality is relatively less influenced by a change in temperature of ink. However, in the case of the piezoelectric system, in particular, in a system involving ejecting ink through use of shear deformation (shear mode) of a piezoelectric element, energy efficiency during ejection is low, and hence, a calorific value of a recording element substrate is large. Consequently, the temperature of ink is likely to rise, which easily influences image quality.
On the other hand, the full-line head is basically required to perform a continuous operation so as to take advantage of the high-speed printing performance. Therefore, in the case where a head is heated excessively, cooling time cannot be provided by suspending a printing operation, unlike a conventional serial head. In the case of performing high-speed printing by forming a full-line head through use of a thermal system or a piezoelectric system of a shear mode, the full-line head is likely to be heated excessively because a calorific value of a recording element substrate is large. As a result, the temperature of ink rises easily.
In view of the foregoing, it has been hitherto proposed to provide a cooling unit in a full-line head through use of forced convection. FIGS. 13A and 13B are schematic views each illustrating an example of a conventional full-line head structure. FIG. 13A is a perspective view of the full-line head, and FIG. 13B is a partial sectional view taken along line 13B-13B of FIG. 13A. As illustrated in FIG. 13B, a flow path 103 for supplying ink is formed in a support member 102. The flow path 103 is connected to an ink tank and a pump (not shown). Ink circulates to flow through a circulation path formed of the ink tank, the pump, and the flow path 103 during head driving. Part of the ink distributed in the flow path 103 is supplied to each recording element substrate 101, and the remaining ink circulates to be supplied to the flow path 103 again. Heat generated in each recording element substrate 101 is discharged to the ink passing through the support member 102. Therefore, a material such as alumina having high thermal conductivity is used for the support member 102.
However, in the configuration illustrated in FIGS. 13A and 13B, that is, a configuration in which the ink is allowed to circulate to be cooled, there is a problem in that the temperature of the ink rises more on the downstream side in the support member 102. The reason for this is that the heat which the ink receives from the recording element substrates 101 accumulates as the ink is distributed to the downstream side in the support member 102, and the total amount of the heat which the ink receives from the recording element substrates 101 increases more on the downstream side. Therefore, in the full-line head, there arises another problem in that density unevenness occurs in printed matter in a width direction of a recording medium. The same problem also occurs in a full-line head in which ink does not circulate. The reason for this is as follows. Even in the case where the flow path in the support member has a dead end, the ink is supplied to the recording element substrate on the downstream side during full-line head driving, and hence, a flow of ink which flows while rising in temperature from the upstream side to the downstream side is formed in the support member.
Patent Literature 1 proposes a head array unit (full-line head) in which a refrigerant fluid is allowed to flow in the head separately from ink so as to cool each recording element substrate. Heat transfer efficiency between the refrigerant fluid and each recording element substrate is set so as to increase from the upstream side to the downstream side of the refrigerant fluid. Thus, a rise in temperature of the recording element substrates on the downstream side of the refrigerant fluid is suppressed, and as a result, a rise in temperature of the ink on the downstream side is also suppressed.
Patent Literature 2 proposes a full-line head in which an insulation member is provided between a circulation flow path in a head and a support plate for recording element substrates. Multiple recording element substrates are mounted on a lower surface of the support plate, and the insulation member made of a plate-like member is adhered to an upper surface of the support plate. A rear surface of the insulation member is fixed to a tank in the head having the circulation flow path. A communication port for supplying ink from the circulation flow path to the recording element substrates is provided so as to pass through the insulation member and the support plate. Due to the presence of the insulation member, heat is prevented from transferring from the recording element substrates to the ink, and as a result, a rise in temperature of the ink on the downstream side is also suppressed.
In the head described in Patent Literature 1, the temperature of recording element substrates on the downstream side of a refrigerant rises as the printing speed becomes higher, and a temperature difference between the recording element substrates increases.
Further, concurrently, a heat discharge amount to the outside of the head increases, and a heat exchanger for cooling the refrigerant is enlarged. Therefore, cooling power as well as head driving power increase.
In the head described in Patent Literature 2, heat transfers between the recording element substrates due to the heat transfer in the support plate and the small thermal spreading resistance, and hence, the temperature of the recording element substrates in the vicinity of a center of the head rises and a temperature difference between the recording element substrates cannot be reduced sufficiently.